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The New Physics and Its Evolution by Lucien Poincare
page 40 of 282 (14%)
can be stayed; and in reality it is now known that the ten-millionth
part of the quarter of the terrestrial meridian is longer than the
metre by 0.187 millimetres. But contemporary physicists do not fall
into the same error as their forerunners, and they regard the present
result as merely provisional. They guess, in fact, that new
improvements will be effected in the art of measurement; they know
that geodesical processes, though much improved in our days, have
still much to do to attain the precision displayed in the construction
and determination of standards of the first order; and consequently
they do not propose to keep the ancient definition, which would lead
to having for unit a magnitude possessing the grave defect from a
practical point of view of being constantly variable.

We may even consider that, looked at theoretically, its permanence
would not be assured. Nothing, in fact, proves that sensible
variations may not in time be produced in the value of an arc of the
meridian, and serious difficulties may arise regarding the probable
inequality of the various meridians.

For all these reasons, the idea of finding a natural unit has been
gradually abandoned, and we have become resigned to accepting as a
fundamental unit an arbitrary and conventional length having a
material representation recognised by universal consent; and it was
this unit which was consecrated by the following law of the 11th July
1903:--

"The standard prototype of the metrical system is the international
metre, which has been sanctioned by the General Conference on Weights
and Measures."

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